1. Field of the Invention
The present invention relates generally to an apparatus and method for spreading channel data in a CDMA communication system, and in particular, to an apparatus and method for spreading channel data in a CDMA communication system using orthogonal transmit diversity (OTD).
2. Description of the Related Art
In order to increase channel capacity, a CDMA (Code Division Multiple Access) communication system spreads channels using orthogonal codes. For example, the forward link of an IMT-2000 system performs channel spreading using orthogonal codes. A reverse link can also perform channel spreading using orthogonal codes through time alignment. An example of an orthogonal code that is typically used is a Walsh code.
The number of available orthogonal codes is determined depending upon a modulation method and a minimum data rate. However, in the proposed IMT-2000 CDMA system, the channels assigned to the users will increase in number in order to improve system performance. To this end, the future CDMA system includes a plurality of common channels and dedicated channels, and assigns the channels to the mobile stations, thereby increasing channel capacity.
However, even in the proposed IMT-2000 CDMA system, an increase in the utilization of the channels limits the number of available orthogonal codes. Further, the reduced number of available Walsh orthogonal codes limits the increase in channel capacity. In an effort to solve this problem, a method has been proposed for using quasi-orthogonal codes for channel spreading codes which have a minimum interference with the orthogonal codes and have a variable data rate.
In the IMT-2000 system, a 1× system uses a spreading code group having a spreading code rate 1, and a 3× system uses a spreading code group having a spreading code rate 3. In the 1× system, the spreading code generator stores spreading codes with a maximum length of 128 and generates a spreading code corresponding to a designated spreading code index to spread code symbols with the generated spreading code. Further, in the 3× system, the spreading code generator stores spreading codes with a maximum length of 256 and generates a spreading code corresponding to a designated spreading code index to spread code symbols with the generated spreading code.
The IMT-2000 system supports a transmit diversity, for which an orthogonal transmit diversity (ODT) scheme is typically used. Further, the IMT-2000 system can support a multicarrier system. Therefore, the IMT-2000 system can either employ or not employ orthogonal transmit diversity for the 1× direct spreading (DS) system according to circumstances. Further, for the 3× system, the IMT-2000 system can support both the multicarrier system and the direct spreading system, wherein orthogonal transmit diversity can be either used or not used for the direct spreading system.
The orthogonal transmit diversity scheme inputs the coded symbols to first and second antennas by dividing, and then divides again the signals input to the first and second antennas into two components respectively by demultiplexing to transmit them via the different antennas. At this point, the symbol rate decreases by half, because the signals input to the first and second antennas are divided into two components by the demultiplexer. Therefore, in order to match the halved symbol rate to the total symbol rate, the divided input symbols are repeated and the pair of symbols (both the original and the repeated symbol) are orthogonally spread. One of the divided components goes to the first antenna, and the second divided component goes to the second antenna. The signal input to the first and second antennas is divided again into two components by demultiplexing, which results in a total of 4 components from the original signal. Then, the 4 components are orthogonally spread with independent orthogonal codes.
In the orthogonal transmit diversity scheme, the respective component symbols undergo repetition before orthogonal spreading. Spreading the repeated symbols with the respective spreading factors is equivalent to spreading one symbol with twice the spreading factors. The receiver then accumulates the chips for two times the spreading factor duration during spreading and multiplexes the accumulated chips. Since spreading the chips using the quasi-orthogonal codes is equivalent to spreading each component chip with twice the spreading factor in the orthogonal transmit diversity scheme, the correlation property of the quasi-orthogonal codes may vary. Actually, when using orthogonal codes of length 256, the correlation for 256 chip duration is ±16 and ±16j. Therefore, any orthogonal transmit diversity scheme should consider the effect of spreading the chips with twice the spreading factor, when selecting the quasi-orthogonal codes for use in the spreading scheme using the quasi-orthogonal codes.
FIG. 1 shows a transmitter using an orthogonal transmit diversity scheme. Referring to FIG. 1, a channel encoder 110 encodes input data into coded symbols, and an interleaver 130 interleaves the coded symbols and provides the interleaved symbols to an adder 120. At this point, a long code generator 100 generates a long code and a decimator 105 decimates the generated long code and provides the decimated long code to the adder 120. The adder 120 adds the decimated long code and the interleaved code symbols, and a demultiplexer 140 demultiplexes the signals input from the adder 120 to the first and second antennas.
The signals demultiplexed to the first and second antennas are input to demultiplexers 150 and 155. The demultiplexer 150 demultiplexes the I-component input signal for the first antenna into I1 and Q1 components, and provides the I1 and Q1 components to symbol repeaters 160 and 162, respectively. Similarly, the demultiplexer 155 demultiplexes the Q-component input signal for the second antenna into I2 and Q2 components, and provides the I2 and Q2 components to symbol repeaters 164 and 166, respectively. The symbol repeaters 160 and 162 repeat their input signal I1 and Q1 two times, respectively. The symbol repeater 164 outputs the I2 signal once and then outputs an inverted input signal. Similarly, the symbol repeater 166 outputs the Q2 signal once and then outputs an inverted input signal. In order to maintain the orthogonality between the first and second antenna signals demultiplexed by the demultiplexer 140, the symbol repeaters 160 and 162 repeat the input symbols in the different manner from the symbol repeaters 164 and 166. Although the symbol repeaters 160 and 162 have a similar operation to the existing symbol repetition, the symbol repeaters 164 and 166 repeat the input symbols in different manner. For example, upon receipt of an input signal ‘1’, the repeaters 164 and 166 output a symbol ‘1’ and an inverted symbol ‘−1’.
Thereafter, a spreader 170 receives the signals output from the symbol repeaters 160 and 162, and at the same time, a spreading code generator 180 generates a spreading code corresponding to an input spreading code index k1 and provides the generated spreading code to the spreader 170. The spreader 170 then spreads the signals output from the symbol repeaters 160 and 162 with the spreading code. Further, a spreader 175 receives the signals output from the symbol repeaters 164 and 166, and at the same time, a spreading code generator 185 generates a spreading code corresponding to an input spreading code index k2 and provides the generated spreading code to the spreader 175. The spreader 175 then spreads the signals output from the symbol repeaters 164 and 166 with the spreading code.
FIG. 2 shows a receiver using orthogonal transmit diversity. Referring to FIG. 2, a despreader 270 receives input data rI1 and rQ1, and at the same time, a spreading code generator 280 generates the spreading code corresponding to an input spreading code index k1 and provides the generated spreading code to the despreader 270. The despreader 270 then despreads the input data rI1 and rQ1 using the spreading code provided from the spreading code generator 280 and provides the despread signals to a multiplexer 250. Similarly, a despreader 275 receives input data rI2 and rQ2, and at the same time, a spreading code generator 285 generates the spreading code corresponding to an input spreading code index k2 and provides the generated spreading code to the despreader 275. The despreader 275 then despreads the input data rI2 and rQ2 using the spreading code provided from the spreading code generator 285 and provides the despread signals to a multiplexer 255.
The multiplexer 250 multiplexes the signals output from the despreader 270 to output a first antenna component, and the multiplexer 255 multiplexes the signals output from the despreader 275 to output a second antenna component. A multiplexer 240 multiplexes the first and second antenna components and provides the multiplexed signals to an adder 220. At the same time, a long code generator 200 generates a long code and a decimator 205 decimates the long code and provides the decimated long code to the adder 220. The adder 220 then adds the decimated long code and the codes output from the multiplexer 240, and a deinterleaver 230 deinterleaves the signals output from the adder 220. A channel decoder 210 decodes the signals output from the deinterleaver 230.
FIG. 3 shows a direct spreading scheme which does not use orthogonal transmit diversity. Referring to FIG. 3, a channel encoder 310 encodes input data into coded symbols, and an interleaver 330 interleaves the coded symbols and provides the interleaved symbols to an adder 320. At the same time, a long code generator 300 generates a long code and a decimator 305 decimates the long code and provides the decimated long code to the adder 320. The adder 320 then adds the decimated long code and the interleaved code symbols, and provides its outputs to a demultiplexer 340. The demultiplexer 340 demultiplexes the input signals into an I-component signal and a Q-component signal. A spreader 370 receives the I-component and Q-component signals, and at the same time, a spreading code generator 380 generates a spreading code corresponding to an input spreading code index k and provides the generated spreading code to the spreader 370. The spreader 370 then spreads the I-component and Q-component signals output from the demultiplexer 340 with the spreading code.
FIG. 4 shows a receiver which does not use orthogonal transmit diversity. Referring to FIG. 4, a despreader 470 receives input data I and Q, and at the same time, a spreading code generator 480 provides the despreader 470 with a spreading code corresponding to an input spreading code index k. The despreader 470 despreads the input data I and Q using the spreading code provided from the spreading code generator 480, and provides the despread signals to a multiplexer 440. The multiplexer 440 multiplexes the despread I and Q components, and provides the multiplexed signals to an adder 420. At this point, a long code generator 400 generates a long code, and a decimator 405 decimates the long code and provides the decimated long code to the adder 420. The adder 420 adds the decimated long code and the codes output from the multiplexer 440, and provides its output signals to a deinterleaver 430. The deinterleaver 430 deinterleaves the input signals and a channel decoder 410 decodes the deinterleaved signals.
The IMT-2000 system having the above spreading scheme supports a multicarrier system. The multicarrier mobile communication system transmits signals at one carrier of a 1.25 MHz band for the 1× system, and transmits the signals at three carriers for 3× system. The respective carriers are assigned independent orthogonal codes. When the 1× system is overlaid with the 3× system, using orthogonal codes of different lengths will cause interference between the systems. Herein, it will be assumed that the 1× system generates a quasi-orthogonal code using a mask function of length 128, and the 3× system generates a quasi-orthogonal code using a mask function of length 256. In this case, since a good correlation property is not guaranteed between a spreading code of length 128 which uses a mask function at a spreading rate 1 and a spreading code of length 128 which uses a mask function at a spreading rate 3 at each 1.25 MHz band, increased interference may occur between a user using a mask function at the spreading rate 1 and a user using a mask function at the spreading rate 3.
When the 1× system uses the quasi-orthogonal code and the 3× system uses the orthogonal code, interference that the quasi-orthogonal code (QOFm+Wk) user of the 1×system, experiences from the orthogonal code (Wj) user of the 3× system can be given by the equation:                                           ∑            i                          T              i                                ⁢                      [                                          (                                                      QOF                                          m                      ,                      i                                                        +                                      W                                          k                      ,                      i                                                                      )                            +                              W                                  j                  ,                  i                                                      ]                          =                                            ∑              i                              T                i                                      ⁢                          [                                                QOF                                      m                    ,                    i                                                  +                                  (                                                            W                                              k                        ,                        i                                                              +                                          W                                              j                        ,                        i                                                                              )                                            ]                                =                                                    ∑                i                                  T                  i                                            ⁢                              [                                                      QOF                                          m                      ,                      i                                                        +                                      W                                          s                      ,                      i                                                                      ]                                      <                          Θ              min                                                          (        1        )            
That is, the interference satisfies an upper limit formula of the correlation for the quasi-orthogonal code. Therefore, in this case, this is not a serious matter. However, when the 1× system and 3× system both use the quasi-orthogonal code, interference that the quasi-orthogonal code (QOFm+Wk) user of the 1× system experiences from the quasi-orthogonal code (QOFn+Wj) user of the 3× system does not satisfy the upper limit formula, as shown in Equation (2) below:                                           ∑            i                          T              i                                ⁢                      [                                          (                                                      QOF                                          m                      ,                      i                                                        +                                      W                                          k                      ,                      i                                                                      )                            +                              (                                                      QOF                                          n                      ,                      i                                                        +                                      W                                          j                      ,                      i                                                                      )                                      ]                          =                                            ∑              i                              T                i                                      ⁢                          [                                                (                                                            QOF                                              m                        ,                        i                                                              +                                          W                                              k                        ,                        i                                                                              )                                +                                  (                                                            QOF                                              n                        ,                        i                                                              +                                          W                                              j                        ,                        i                                                                              )                                            ]                                =                                    ∑              i                              T                i                                      ⁢                          [                                                (                                                            QOF                                              m                        ,                        i                                                              +                                          QOF                                              n                        ,                        i                                                                              )                                +                                  W                                      s                    ,                    i                                                              ]                                                          (        2        )            
In this case, the mutual interference between the channels increases.
Therefore, when using the quasi-orthogonal codes of spreading code groups having different lengths, the mobile communication system stores the spreading codes of different lengths, and thus increases the hardware complexity. Further, using the spreading codes having different spreading rates in the overlay scheme deteriorates the interference property between two users thereby causing performance degradation.
FIG. 5 shows a transmitter for a 3× multicarrier system. Referring to FIG. 5, a channel encoder 500 encodes an input signal into coded symbols, and an interleaver 505 interleaves the coded symbols. A long code spreader 510 spreads the interleaved symbols with a long code output from a long code generator 515. A demultiplexer 580 demultiplexes the spread signals into three components, each of which is divided again into I component and Q component, and provides the I and Q components to spreaders 520, 522 and 524.
When the spreader 520 receives the signals from the demultiplexer 580, a spreading code generator 540 generates a spreading code of length 256 corresponding to an input spreading code index k indicating a channel assigned to the user, and provides the generated spreading code to the spreader 520. The spreader 520 spreads the long code spread signals at a chip rate of 1.2288 Mcps by operating each symbol of the input signal with a specified number of chips (256/2n, 0≦n≦6) of the spreading code. When the spread signals are input to a PN spreader 530, a short PN code generator 550 generates a short PN code and outputs the generated short PN code at a chip rate of 1.2288 Mcps. The PN spreader 530 PN spreads the input signals with the PN codes output from the short PN code generator 550. Since the other spreaders and spreading code generators have the same operation, a detailed description will not be given in order to avoid duplication.
FIG. 6 shows a receiver for the 3× multicarrier system. Referring to FIG. 6, when the spread signals are input to a PN despreader 630, a short PN code generator 650 generates a short PN code and outputs the generated short PN code at a chip rate of 1.2288 Mcps. The PN despreader 630 operates the input signals and the short PN code on a chip unit basis to output PN despread signals.
When the PN despread signals are input to a despreader 620, a spreading code generator 640 generates a spreading code of a maximum length 256 corresponding to an input spreading code index k indicating a channel assigned to the user, and provides the generated spreading code to the despreader 620. The despreader 620 then operates on each symbol of the PN despread signal with a specified number of chips (256/2n, 0≦n≦6) of the spreading code, and accumulates the signals. The despread signals from the despreader 620 are provided to a multiplexer 680. In the same manner, the signals input to PN despreaders 632 and 634 are provided to the multiplexer 680 after despreading. The multiplexer 680 then multiplexes the input signals despread through three different paths in the reverse order of signal demultiplexing performed in the transmitter. When the multiplexed signals are input to a long code despreader 610, a long code generator 615 generates a long code. The long code despreader 610 despreads the multiplexed signals with the long code output from the long code generator 615. A deinterleaver 605 deinterleaves the long code despread signals and a channel decoder 600 decodes the deinterleaved signals.
In the CDMA communication system using orthogonal transmit diversity, even though the same symbol is repeated two times when spreading the signals transmitted to the respective antennas, it is undesirably necessary to spread the symbols using the orthogonal codes according to the spreading rates of the respective symbols.